1. Field of the Invention
The present invention relates, in general, to sensors and, in particular, to enzymatic electrochemical-based sensors.
2. Description of the Related Art
The use of enzymatic electrochemical-based sensors that employ a redox mediator (e.g., ferrocene) and a redox enzyme (such as glucose oxidase) in conjunction with an electrode(s) for the determination of an analyte in a liquid sample has become of heightened interest in recent years. Such enzymatic electrochemical-based sensors are believed to be particularly suitable for continuous or semi-continuous monitoring of analytes (for example, glucose) in bodily fluid samples (e.g., blood or interstitial fluid samples). For example, enzymatic electrochemical-based glucose sensors employing a redox mediator, a redox enzyme and a working electrode can determine (i.e., measure) glucose concentration using relatively low potentials (e.g., less than 0.4 V vs SCE), thereby limiting any interfering responses, at the working electrode. For a further description of enzymatic electrochemical-based sensors, see, for example, U.S. Pat. Nos. 5,089,112 and 6,284,478, each of which is hereby fully incorporated by reference.
Proteins surrounding a redox enzyme's redox center can preclude direct transfer of electrons from a redox enzyme to an electrode of an enzymatic electrochemical-based sensor. Therefore, in typical enzymatic electrochemical-based sensors, a redox mediator is employed to facilitate electron transfer between the redox enzyme(s) and an electrode(s) of the electrochemical-based sensor. In such a circumstance, the redox enzyme cycles between oxidized and reduced states, driven by the presence of analyte, the redox mediator and a surface of the electrode. The net result of such cycling is that electrons are either accepted or donated at the surface of the electrode while the redox enzyme essentially maintains its original oxidation state and catalytic characteristics.
For enzymatic electrochemical-based sensors that require long term stability, such as continuous or semi-continuous electrochemical-based glucose sensors, it is essential that both the redox mediator and the redox enzyme do not leach away from the vicinity of the electrode. Therefore, it is not desirable to employ readily leachable redox mediators (such as readily leachable ferricyanide, benzoquinone and low molecular weight quinone derivatives, ferrocene, low molecular weight ferrocene derivatives, ruthenium complexes and osmium complexes) in enzymatic electrochemical-based sensors. In addition, if the redox mediator is a substance that is harmful to humans or other subjects, leaching of the redox mediator into a human's or other subject's body is undesirable and thus to be avoided.
Furthermore, the redox enzyme and redox mediator of enzymatic electrochemical-based sensors must be able to favorably interact with one another and the redox mediator must be able to exchange electrons with an electrode of the enzymatic electrochemical-based sensor. In other words, the activity of both the redox enzyme and redox mediator should be maintained while inadvertent leaching is prevented.
To prevent redox mediator leaching, certain chemical compositions wherein redox mediators are chemically attached to redox enzymes have been proposed for use in electrochemical-based sensors. The redox enzymes of such chemical compositions can, however, suffer from a deleterious decrease in enzyme activity.
Alternatively, redox mediators have also been attached to water-insoluble synthetic polymer chains, such as polysilozanes, in order to prevent leaching. However, such chemical compositions suffer from low flexibility, and thus a reduced mediation activity, due to their hydrophobic nature. In addition, the attachment of redox mediators to water insoluble synthetic polymer chains does not directly address the need to prevent leaching of the redox enzymes employed in enzymatic electrochemical-based sensors.
Still needed in the field, therefore, is a chemical composition for use in enzymatic electrochemical-based sensors that prevents inadvertent leaching of both redox mediators and redox enzymes from the vicinity of the electrochemical-based sensor's electrode while maintaining adequate activity of the redox mediator and redox enzyme.